(349o) A DFT and Microkinetic Modeling Study of Confinement Driven Diels-Alder Reactions in Acidic Zeolites

Diels-Alder (DA) reactions are a well-studied class of C-C bond formation reactions whereby conjugated dienes can react with substituted olefins to form cyclic products. They may proceed in the absence of any catalyst, but Brønsted/Lewis-acids have shown to enhance their reaction rates. Nevertheless, controlling the selectivity of specific stereo-/regio-chemistries can be challenging and DA products may unfavorably further couple to form bicyclic species. Increasingly, zeolites are being explored as DA catalysts, wherein the presence of acidic-sites and confinement offer means to tune the kinetics and product selectivity. Previous mechanistic analyses have largely focused on the ethylation of furans to form aromatics, but the kinetics and regioselectivity of other classes of DA reactions have not yet been explored.

We here study the mechanism and energetics of the DA reaction between isoprene and ethylene in HZSM-5 by using density functional theory; and infer their implications on the reaction rates and product yields using microkinetic modeling. The cross-coupling between ethylene and isoprene forms a C7 product and self-coupling of isoprene forms a variety of C10 products. Our results suggest that the DA reaction rates are increased ~10,000 fold on Brønsted-acid sites of HZSM-5 relative to the uncatalyzed reactions at 1 atm & 373 K. An energy decomposition analysis suggests that this rate enhancement is driven by favorable dispersion interactions imparted by the framework on the transition states. The C-C coupling steps and desorption of the self-coupled products are both kinetically controlling, leading to a negative apparent order for the C7 product with respect to isoprene. This indicates that rate/selectivity of C7 products can be enhanced by modulating the reactant feed ratio.

This talk will discuss the atomic scale potential energy diagram of DA and competing reactions of the isoprene-ethylene system within HZSM-5 (and other metal exchanged ZSM-5) and their kinetics via microkinetic modeling.